1. Field of the Invention
The invention relates generally to casters having one or more wheels of specific construction, and more particularly to a caster wheel having a resilient tread with engineered rim or hub interface to more effectively accommodate a wide range of loading stresses.
2. Description of Related Art
A caster wheel is single, double, or compound wheel that is designed to be mounted to the bottom of a larger object, or vehicle, so as to enable that object to be easily moved. The term “caster wheel” as used herein is intended to apply to both driven and un-driven configurations, although un-driven or free-wheeling configurations are more common. High capacity, heavy duty casters are used in many industrial applications, such as platform trucks, carts, assemblies, and tow lines in plants.
A standard caster wheel has a center rotating hub (with or without a bushing or bearing) and a compliant tread material applied about its outer periphery as a rolling contact surface. The outer diameter of a caster wheel affects how easy it is for the caster to be able to move across rough or irregular surfaces. Large diameter caster wheels are able to bridge wide gaps, such as between an elevator door and an elevator car. In situations where heavy loads need to be transported on casters, load capacity may be increased by using wider wheels with more ground contact area. However, when rotating in-place a wide swivel caster, the center part of the wheel-to-ground contact patch rotates slower than the regions further out to the sides. This difference in rotation speed across the base of the wheel contact patch causes wide wheels to resist rotation around the point of swivel, a resistance which increases as weight loading increases.
An alternate way to increase load capacity while limiting swivel-rotation resistance is to use multiple narrow wheels in tandem on the same wheel axis, as shown for example in FIG. 1. Each wheel of a dual-wheel swiveling caster has a comparatively narrower ground contact patch (i.e., footprint) than a single wide wheel and each wheel is able to rotate independently at a different rate, so there is less resistance to turning in place on the swivel. There are several scenarios that dictate the use of a dual-wheel caster in an industrial application.
A first one of these scenarios is when the anticipated load cannot easily be carried by one wheel. When the anticipated load to be transported exceeds the load rating of one wheel then a dual-wheel configuration can be an excellent solution. A second scenario occurs when there is a need to reduce the height of the cart or platform. One way to reduce the height of a cart or platform that is fitted for casters is to spread the load over more wheels. When the load is spread over twice (2×) the number of wheels, the diameter of the wheel can be reduced as the calculated load on each wheel is halved. The use of a dual-wheel caster will generally reduce the overall height by approximately 30% while carrying the same load. Thirdly, dual-wheel casters are necessary when there is the need to reduce per square inch of floor loading. Commonly in applications where there is restriction on per square inch loading of floors (due to structural concerns or surface concerns) the load can be proportionally spread across a greater wheel surface. Still further, dual-wheel casters enable easier swiveling when a cart is fully loaded. Generally stated, the heavier the load is on each wheel (greater load per square inch) the more force it will take to swivel a caster assembly. The use of dual-wheels halves the per-wheel load and thus can allow assembly swiveling to take place with less force. As a dual-wheel caster assembly swivels, one wheel will often turn clockwise while the other wheel rotates counter clockwise. In effect, the vertical swivel axis is located between the two wheels, thereby reducing wheel scrubbing and making the swiveling of heavy loads simpler.
Certain uses may require a caster wheel to have a resilient tread material around the outer perimeter of the wheel. The tread can be made of many elastomeric materials and take on different shapes, such as a pneumatic tire, a coating of polyurethane or over-molded elastomer. The hardness of a caster wheel tread affects ease of rolling, traction, durability, shock absorption, and noise. Hard wheels are easier to roll on smooth surfaces, but are noisier and provide less traction. Soft wheels are easier to push on rough surfaces, quieter, protect the floor, absorb shock and provide better traction, but have a lower load capacity. This presents a problem in situations where a heavy load needs to be transported with the positive effects of soft wheels, for example, good traction, shock absorption and floor protection. If an elastomeric tread is omitted altogether to increase load capacity, then the positive aspects of the tread are also removed.
The radial thickness of the tread will affect the rolling characteristics of a caster wheel. All things being equal, thin treads are easier to push while thick treads have better shock absorption. Again, in industrial situations where ease of transportation by pushing combined with good shock absorption is key, the design engineer is frequently left with a compromise or trade-off between low rolling resistance and good shock absorption.
Furthermore, the working life of an elastomeric tread tends to define the working life of a caster wheel assembly. That is, the tread tends to be the primary wear part of a caster wheel assembly. When the tread life is near its end, the entire caster assembly is typically replaced or at least the worn wheel is replaced. One factor contributing to accelerated tread wear is shear stresses propagating through the elastomeric tread material. Elastomeric materials of the type used for caster wheel treads tend to be strong and durable under compression, but substantially more fragile in shear. Working life is also diminished by excessive heat build-up in the tread. Heat is generated while the caster wheel is rolling and the tread is being deformed. Tread thickness has an effect on heat dissipation; thicker treads tend to dissipate heat more slowly into the hub (a heat sink), and thus accelerate wear.
There is a need for a caster wheel that behaves like a thin/hard tread wheel, but has the good characteristics of a thick/soft tread. More still, such a wheel should be readily adaptable to multi-wheel applications that permit a heavy load to be transported on a soft and thin tread material. And even more specifically, there is a need for an optimized tread design with improved load carry capabilities with the ability to better distribute load stresses so as to remove or at least reduce shear effects. Still further, there is a need for a tread design that facilitates heat removal into the hub.